Teardrops n Tiny Travel Trailers

What is the consensus on joining 2" thick foam cutoffs to make larger panels?

My plan is for a 5 x 5 x 10 foamie of approximately this outline:


I will have a lot of cutoffs of significant size. I'm not concerned with joining the pieces to make up the sides as the joints will be under compression, but what about the top? There will be 7 pieces 5' 4" long to make up the top. does anyone see any issues using joined pieces to span the top?

I would use a hot wire jig to cut 1" x 2" grooves in the edges, then use 3/4" x 3 3/4" foam as splines, then use Great Stuff to fill in the gaps and glue them together. Like this:



and this (3/4" x 2 3/4" spline):



I know the main strength of the shell comes from the canvas/tb2, and I won't have a lot of weight on the top, so I think it will be OK.

So what does this community of TTT gurus think?

NOT from experience but I'd sure avoid pieces on the roof that are not full width. Just the roof it would seem like 6 sheets versus 4 and I'll squeeze a nickle till the buffalo poops but this seems like not the place to do it. Use the cutoffs for bulkhead and cabinets and specifically in or under floor. Pieces together walls probably only create appearance issues if any. I think my time would be worth more to me than the savings for a few sheets.

But listen to others for any who have done what you propose.

I would not bother with the fancy splined joints. They aren't needed, they don't add any strength, and they would add unnecessary complication. The foam is only as strong as the foam is and splining with more foam... other than the added glue... won't make it any stronger.

Butt joints are fine. It is the skins that do the work... or embedded wooden spars. There has been talk (and perhaps an example or two) of people using thin 1/4 plywood spars glued in between foam butt joints with the plywood vertical, just like traditional spars. It really depends on the width of your cabin and the amount of arch in your profile as to whether you might want to consider supplementing the foam and skins with more structure... that and whether you are planning on having a roof rack, roof mounted solar panels, etc.

If you are thinking that the splines will make the joints easier to align, that may be so, but the foam has enough variance in thickness... and you will want to round the corners over anyway... that you will still end up sanding/trimming to even things up. Do a small scale test on a few scrap chunks; one w/o splines and one with.

I'd be more likely to clamp a couple of temporary 2x4's on (perhaps covered with a release agent) to use as a clamping alignment aid during the glue ups.

If you still think splined joints are a good idea, consider using thin ply and router or table saw the slots. still won't add much strength until the skins are on (essentially creating a weak spot that would be easy to blow out until the skins go on). I say this, using a router or table saw, because I have never had much luck getting crisp clean accurate cuts with a hot wire. It always tends to "hog out" or scarf resulting in a loose/less than perfect fit... which would kind of defeat the purpose of the spline.

I know some people have gotten really good results with hot wire cutting, but again, test before you decide. I think those experts are the exception, not the rule. $0.02

I guess what I am really trying to say is that the foam is easy to manipulate, shape, cut, sand, glue, etc... have a shop vac and dust mask/respirator ready... and you will be doing plenty of these tasks anyway, so don't bother adding the complicated joints. You would likely be regretting the effort that it takes before you get done.

A biscuit cutter or wing cutter in a router with 1/8" splines will make long joints easier to align when clamping is difficult or impossible. But any joint stronger than the surrounding foam is a waste of time.

Even the glue is stronger than the foam .

The thing is , if you need STRONGER , make the Foam THICKER ! Twice as Thick is 8 times as strong . True!!! ( even without the Timber framing )

We made our current trailer by joining large sheets of Hot wire cut EPS just with nothing more than TB2 glue and Great Stuff . Once canvas covered, it makes a formidable structure that has survived for three years + living outside now .

OK ... forget the splines. What about using 1" foamboard doubled up with joints offset instead of 2" foam butt joined.


This method would allow me to make foam panels that are basically 5 x 10 that I could trim to the side profile, and full 5'4" length panels for the top.

What do you all think? Would the added strength be worth the extra time and TB2? or is it really true that butt joints are good enough?

That would work perfectly, staggering the seams and bonding the sheets.
You might be surprised how much glue you can use up in a big surface lamination, and adding extra layers make you buy more glue.

2" foamboard is currently $29, 1" is $19.
Just buying double the number of sheets will increase my cost by about 30% plus the added glue.
But it will probably allow me to utilize much more of the cutoffs and will probably reduce the number of sheets by about the same amount.
I'll play around with laying out a cut list to see how much of the cutoffs I can use.
It may just end up costing a bit more in glue and time, but being 'semi-retired' I have a lot of that, right?

I haven't had or seen any issues with butt joints on my camper, that's using 2" foam. The gorilla glue is really strong and then with glued canvas on each face it is pretty well fixed. Do a test. Even a butt joint that it taped with packing tape and no glue in the join approaches the strength of a solid piece.
Laminating would be a job for sure, not worth it in my opinion.
The one thing that I could see happening with yours is some sag on the largest and most horizontal sections. Sagging wouldn't be from the joins but simply because foam is flexible and prone to it, you know I'm sure.

My trailer is 78" across. I did introduce some camber to the roof during glue up (put some poles underneath) and I have a floor to ceiling wardrobe on one side that keeps it from sagging. The other side isn't supported (floor to ceiling) but is doing okay.

Homebrewer25 wrote:What do you all think? Would the added strength be worth the extra time and TB2?

No, not worth the time and materials.

The foam is just a spacer. It doesn't need or have much strength in bending, shear or point loaded compression.

The strength comes from the fact that it can resist compression over a large area, so it is a nice light weight material to use for stabilizing your skins at a set distance apart. The skins provide all of the tensile strength for the panel that resist bending and/or snapping, as well as puncture resistance, and shear.

The further apart you hold the skins, the greater the bending resistance, making a stiffer panel. The thicker and stronger the skin fibers, the more puncture and shear resistance.

The foam is just a spacer. So long as it doesn't crack or break while you are handling it to make your panels, that's all the strength it needs to have.

Think unity of construction, not individual component strength. Think composite structure, not pieces and parts.

Thanks for all the input.
So I'll go ahead with 2" and forget all this nonsensical worrying about the strength. You guys with foamie experience are worth listening to.

Nice looking trailer, Don.
KC, I'm about 60 pages into your TPCE build ... all I can say is WOW!!

I've got an old 32 x 32 drafty dirt floor tobacco barn in which I've built my (heated) 16 x 24 wood shop at one end. I'm thinking of having a nice level 12x16 concrete slab poured in the center bay of the other end that would be a great place for a TTT build. Although the barn wouldn't be heated, I can build my panels/pieces/parts inside and wait until warm weather to assemble them outside.

I've got a friend that I showed my plans to and he says he wants me to make him one too.
I'm not sure that I want to take that on, but it would be the perfect opportunity to build one for him, then one for me without the mistakes that I'm sure to make on his.

I'm getting closer to starting my build, so you guys will probably get sick of all my questions.

Ok ... wait a minute ... another idea what are your thoughts on adding a 3/4" x 2" poplar stick between the joints on the roof? Seems like that would stiffen it enough to avoid any sagging until I get it canvassed.

I like the idea of using some wood.
Have a look at the Big Pink build to see where foam was joined:

http://tnttt.com/viewtopic.php?f=55&t=44091

and wood spars were added to support the foam:

http://tnttt.com/viewtopic.php?f=55&t=44091&start=220

Because I was building from the inside out, I wanted to put my 5mm okoume plywood ceiling skin on top of the walls first before adding the foam (similar to Steve Fredrick's method).

On dry fit the weight of the plywood caused itself to sag, so I used roof spars glued to the top of the ceiling panel before installing.

With just foam, or a canvas skinned ceiling, I may have been able to stick the ceiling skin onto the foam first and then wrap the foam over (although not on the tighter radius areas) without sagging. I don't think i could have bent 2 inch foam that far, but with two layers of 3/4 it may have worked. (But I don't recommend that either because laminating large panels of foam is a pain and takes much more effort than just kerfing thicker stock.)

When you get to that point in your build (...or even before do a mock up) lay a large piece of foam across and see if it sags, then decide.

For non sagging roofs , how about a kerf cut or two , full length down the middle on the underside , top slightly elevated (kerf) in the center down the middle . ? Perhaps a rib or two for more strength ...